Improvements relating to infra-red radiation sources

Abstract: There is described an infra-red radiation source comprising an electrically conductive element (2) formed of a plurality of carbon fibres and connection means (3,4,5) for connecting the electrically conductive element (2) across an electrical power supply, said connection means (3,4,5) including at least one support member (3) formed of carbon and secured to one end of the electrically conductive element (2).

Description: The present invention relates to infra-red radiation sources and in particular to those sources comprising an electrically conductive element formed of a plurality of carbon fibres. Infra-red radiation sources are used as heat sources in commerical process ovens, domestic cooker hot plates and ovens, and radiant energy electrical heaters. In each of the aforementioned applications there is a requirement for a higher degree of control of the heat output by such sources. To attain this higher degree of control the heat output from the source should rapidly reach an equilibrium value for a predetermined and constant electrical power input. Furthermore, the electrical resistance of the source should not vary to any great extent as it heats from room temperature to its maximum operating temperature. It is also desired that the source be capable of operating effectively through a wide range of power outputs which in turn necessitates a wide range of element temperatures such as, for example, between 600°C and 1800°C. There is in addition a further requirement for the processing of varied pigmented surfaces such that the source should have an enhanced emissivity at wavelengths much longer than those of the visible spectrum. In the past a number of designs of infra-red radiation source have been proposed in which various starting materials are used for forming the or each electrically conductive element. Most of these starting materials have, after processing, yielded brittle elements which are difficult to handle. It has been found however that the use of carbon fibre fabrics and linear tapes impregnated with certain resins and thermoplastics such as epoxy allow the fabrication of elements having the required degree of flexibility. One design comprising an electrical conductor made up of carbon fibres is described in WO-A-90/16137. The carbon fibre element is supported at opposite ends between two members which are adapted so as to facilitate the connection of the element across an electrical power supply. The supporting members are described as being metal and in practice have been formed of such metals as tungsten, molybdenum, nickel or steel. However, one of the problems associated with carbon fibre infra-red radiation sources is the tendency for the element to become sufficiently degraded within the region of the metal supporting members as to result in the failure of the radiation source after only a few tens of hours of operation. This degradation comes about as a result of either migration of carbon atoms from the conductive element into the metal of the or each supporting member or as a result of a reaction between the carbon and the metal to form the appropriate metal carbide. In either event carbon atoms are removed from the element resulting in its eventual collapse. As the carbon atoms are removed there is a tendency for the temperature of the element to increase which in turn only serve to exacerbate the mechanisms by which the carbon is lost. In IBM Technical Disclosure Bulletin Vol. 29, No. 4, 01 September 1986, p. 1552-1554, there is disclosed generally an infrared lamp for vertical burn in which a filament which can stretch significantly when heated to operating temperature is supported by transverse elements which are positioned by dimples formed in the glass of the infrared lamp. According to the present invention, there is provided an infra-red radiation source comprising a housing formed of a material transparent to infra-red radiation, an electrically conductive element located within the housing and formed of a plurality of carbon fibres, connection means for connecting the electrically conductive element across an electrical power supply and restraining means for limiting unwanted movement of the conductive element with respect to the housing; said restraining means including one or more formations arranged in one or more opposed pairs on an internal surface of the housing, and further including one or more yokes, the or each yoke being retained by a respective pair of said formations, wherein said unwanted movement is limited by the engagement of the electrically conductive element with the one or more yokes, and with the or each yoke being formed of a plurality of carbon fibres or of graphite paper. Preferably the formations comprise one or more pairs of diametrically opposed pinches. Advantageously the or each yoke is formed of graphite paper and includes a tantalum shim. Advantageously said at least one yoke includes at least one carbon fibre spacer. Preferably said at least one carbon fibre spacer is woven through the electrically conductive element and extends in a direction generally transverse thereto. Advantageously said carbon fibre spacers are constrained with respect to the housing by engagement with the formations. Advantageously said housing is filled with a chemically inert and thermally insulating gas. Preferably said gas is at sub-atmospheric pressure. According to the present invention there is provided a method of making an infra-red radiation source comprising the steps of forming an electrically conductive element from a plurality of carbon fibres, disposing the electrically conductive element within a housing formed of a material transparent to infra-red radiation, providing the electrically conductive element with restraining means to limit unwanted movement of the electrically conductive element with respect to the housing and to secure the electrically conductive element for connection across an electrical power supply, wherein said step of providing the electrically conductive element with restraining means to limit unwanted movement of the electrically conductive element with respect to the housing comprises providing the internal surface of the housing with one or more formations arranged in one or more pairs, and providing one or more yokes, the or each yoke being retained between a respective pair of said formations, and locating the electrically conductive element with respect to said one or more yokes, each said yoke being formed from a plurality of carbon fibres or carbon paper. A number of embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:- Figure 1 is a schematic perspective view of an infra-red radiation source; Figure 2 is a cross-sectional side view of an example of a support member for use in an infra-red radiation source with which the present invention can be used; Figure 3 is a cross-sectional side view of a prior art arrangement whereby the electrically conductive element may be located with respect to a surrounding tube; Figure 4 is a cross-sectional side view of a first embodiment of the present invention whereby the electrically conductive element may be located with respect to a surrounding tube; Figure 5 is a cross-sectional end view of the arrangement of Figure 4; Figure 6 is a cross-sectional side view of a second embodiment of the present invention whereby the electrically conductive element may be located with respect to a surrounding tube; and Figure 7 is a cross-sectional end view of the arrangement of Figure 6. Referring to Figure 1, the infra-red radiation source may be seen to comprise a tube 1 of material which is transparent to infra-red radiation, such as for example a ceramic material such as quarzglas or fused silica. The tube 1 contains an electrically conductive element 2 in the form of a flat or coiled strip formed of carbon fibres which are coated with and bonded by the carbon residue of a carbonised resin. At each end of the strip 2 there is provided a respective one of two connectors 3 which are both mechanically and electrically connected to the strip 2. Each connector 3 is connected to a respective electrical conductor 4 which is in turn connected to a respective electrical feed through lead 5 which passes through an otherwise closed end of the tube 1. The electrical feed through leads 5 are adapted so as to be connectable across a suitable electrical power supply such that in use the strip 2 may be caused to emit infra-red radiation. Turning now to a more detailed consideration of the connectors 3 by which the electrically conductive element 2 is connected to the electrical conductors 4, the or each connector 3 is formed of a metal, such as copper, through which carbon does not diffuse or of a metal coated with another metal through which carbon does not diffuse. In such an arrangement the metal connectors 3 may be either alloyed or coated with a material that will both wet the surface of the carbon fibres of the strip 2 and provide a good electrical contact between the strip and the or each connector 3. One way in which this might be achieved for a copper connector is to alloy the copper with 1% chromium. Another example of a metal that is capable of wetting both copper and the carbon fibres of the electrically conductive element is gold which may thus be used at an interface between the two materials. Irrespective of the metal coating that is used the coating may be applied to the ends of the electrically conductive element 2 either by an electroplating process or by the application of a metal based paint which is subsequently heated to drive off the solvent and/or organic carrier to leave the metal deposit. In Figure 2, there is shown an example of how the or each connector 3 can be constructed. Each connector 3 comprises a pair of carbon blocks 6, 7 disposed on either side of the electrically conductive element 2 and which are secured together so as to retain the element therebetween. By pressing the two carbon blocks 6 and 7 together it is possible to form a carbon-carbon compression bond between the blocks and the carbon fibres of the strip 2. If, in addition, the blocks 6, 7 and the strip 2 are both heated whilst being pressed together then additional bonding may occur as a result of the melting and subsequent carbonising of the carbon-based resin used to coat the carbon fibres in either event the increased thickness of carbon at the or each connector 3 when compared with the central region of the strip 2 provides the two fold advantage of reducing the heat generated within the vicinity of and conducted to the electrical conductors 4 whilst at the same time providing additional strength for mechanical connection. In Figure 2 this mechanical connection is provided by means of a nut and bolt or rivet 8 which passes through a through-bore 9 provided in each of the carbon blocks 6, 7 and which extends in a direction substantially perpendicular to the plane of the carbon fibre strip 2. Other forms of connection of the element 2 are disclosed in the art, including WO 94/28693. In Figure 3, the quarzglas tube 1 is shown provided with restraining means for limiting unwanted movement of the conductive element with respect to the housing. The restraining means is provided at intervals along the length of the tube 1 with a plurality of pairs of diametrically opposed pinches 39. One such pinch is shown in Figure 4 to comprise n acruate recess 40 provided in the wall of the tube which is defined by two radially inwardly projecting indentations 41 and 42. In the arrangement shown in Figure 3 the carbon fibre strip 2 is mounted with respect to the tube 1 in such a way that the strip and the pinches 39 are substantially co-planar. In this way the carbon fibre strip may be received within the arcuate recess 40 of each of the pairs of diametrically opposed pinches 39. Thus the electrically conductive element may be orientated with respect to the tube 1 and constrained from unwanted lateral or rotational movement whilst at the same time being allowed to expand and contract in a longitudinal direction. In the first embodiment of the present invention shown in Figures 4 and 5 the quarzglas tube 1 is provided at intervals along its length with a plurality of pairs of diametrically opposed pinches 39. However, instead of retaining the electrically conductive element 2 as in Figures 2 and 3, the pinches 39 serve to retain a carbon fibre or graphite paper yoke 43 and it is this yoke that serves to prevent excessive lateral or rotational movement of the electrically conductive element whilst at the same time allowing for expansion of the element in a longitudinal direction. In such an arrangement the yoke 43 may be formed from graphite paper or resin-impregnated carbon fibre bonded together at a pressure of approximately 6Kg and at a temperature of between 300 and 400°C. If the yoke 43 is formed of graphite paper, then the yoke may be further supported by a tantalum shim which may not only provide the yoke with an increased rigidity but may also act as an oxygen getter. In a second embodiment of the present invention shown in Figures 6 and 7 a plurality of carbon fibre spacers 44 is woven through the electrically conductive element 2 at intervals along its length in such a way that the spacers extend in a direction substantially co-planar with but transverse to the electrically conductive element. Each of the carbon fibre spacers 44 is preferably of sufficient length such that its opposite ends are capable of engaging opposing regions on the walls of the quarzglas tube 1. In this way the spacers 44 may simply act to locate the electrically conductive element 2 with respect to the tube 1. In an alternative arrangement the tube 1 may be provided at intervals along its length with a plurality of pairs of diametrically opposed pinches 39 capable of receiving the opposite ends of the spacers 44. In either case the fact that the electrically conductive element 2 is formed of a plurality of carbon fibres which extend longitudinally of the element means that the element is capable of a slight longitudinal movement with respect to the spacers 44 which can be utilised to allow for contraction and expansion of the element. Having inserted the electrically conductive element 2, the tube 1 is sealed and can either be filled with a chemically inert gas of low thermal conductivity, such as argon, at sub-atmospheric pressure, or evacuated. In the former case the filling pressure of the gas is chosen so that the infra-red transparent tube 1 is not unduly stressed throughout the operating temperature range of the source while the specific gas that is used is chosen to prevent deterioration of the surface of the carbon fibres of the strip 2 by oxidation and to minimise heat transfer from the strip 2 to the tube 1. Although the illustrated carbon fibre source has been described as having an infra-red transparent tube 1 which encloses and surrounds the strip 2 in an inert atmosphere or a vacuum, any method of protecting the strip 2 from oxidation may be used. One such method might be the application of a protective coating capable of withstanding the high temperature of operation of the source. One such coating might comprise silicon carbide (SiC). Alternatively the surface of the strip 2 may be doped with boron.

Claims: An infra-red radiation source comprising a housing (1) formed of a material transparent to infra-red radiation, an electrically conductive element (2) located within the housing and formed of a plurality of carbon fibres, connection means (3) for connecting the electrically conductive element across an electrical power supply and restraining means for limiting unwanted movement of the conductive element with respect to the housing; said restraining means including one or more formations (39) arranged in one or more opposed pairs on an internal surface of the housing, and further including one or more yokes (43, 44), the or each yoke being retained by a respective pair of said formations, wherein said unwanted movement is limited by the engagement of the electrically conductive element with the one or more yokes, and with the or each yoke being formed of a plurality of carbon fibres or of graphite paper. An infra-red radiation source as claimed in Claim 1, wherein the formations comprise one or more pairs of diametrically opposed pinches (39). An infra-red radiation source as claimed in either of Claims 1 and 2, wherein the or each yoke is formed of graphite paper and includes a tantalum shim. An infra-red radiation source as claimed in any of Claims 1 to 3, wherein at least one yoke includes at least one carbon fibre spacer (44). An infra-red radiation source as claimed in Claim 4, wherein said at least one carbon fibre spacer is woven through the electrically conductive element and extends in a direction generally transverse thereto. An infra-red radiation source as claimed in any of Claims 1 to 5, wherein said housing is filled with a chemically inert and thermally insulating gas. An infra-red radiation source as claimed in Claim 6, wherein said gas is at sub-atmospheric pressure. An infra-red radiation source as claimed in any of Claims 1 to 6, wherein said housing is sealed and evacuated. A method of making an infra-red radiation source comprising the steps of forming an electrically conductive element (2) from a plurality of carbon fibres, disposing the electrically conductive element within a housing (1) formed of a material transparent to infra-red radiation, providing the electrically conductive element with restraining means to limit unwanted movement of the electrically conductive element with respect to the housing and to secure the electrically conductive element for connection across an electrical power supply, wherein said step of providing the electrically conductive element with restraining means to limit unwanted movement of the electrically conductive element with respect to the housing comprises providing the internal surface of the housing with one or more formations (39) arranged in one or more pairs, providing one or more yokes (43, 44), the or each yoke being retained between a respective pair of said formations, and locating the electrically conductive element with respect to said one or more yokes, each said yoke being formed from a plurality of carbon fibres or carbon paper. A method as claimed in Claim 9, wherein said step of providing the electrically conductive element (2) with restraining means to limit unwanted movement of the electrically conductive element with respect to the housing comprises forming at least one of the yokes as a spacer (44) of carbon fibre and weaving the at least one spacer through the electrically conductive element so as to extend in a direction substantially transverse thereto.

Inventors: DEXTER AMOS CHRISTOPHER, JONES WILLIAM, DEXTER, AMOS CHRISTOPHER, JONES, WILLIAM
Applicants: EXSIHO LTD, EXSIHO LIMITED
Application year: 1994
Main classification: H05B3
IPC codes: H05B3